1. Field of the Invention
The present invention relates to a cylindrical inner surface scanner which is applied to CTP (computer to plate: direct prepress system) for printing/prepressing. The "CTP" is an apparatus for preparing a printing plate by directly forming halftone dot images on a plate material on the basis of digital image signals integrating characters, figures and images, without forming a halftone dot film.
2. Background of the Invention
As known in the art, a cylindrical inner surface scanner is an apparatus having a cylindrical inner surface drum, a photosensitive material which is mounted on the inner surface of the drum, a scanning head which is moved in a subscanning direction along a central axis of the drum, a deflector for main scanning which is arranged on the scanning head and rotated about the central axis, and an imaging lens, and is adapted to deflect a light beam which is modulated by an image signal or the like by the deflector through the aforementioned lens, thereby scanning/exposing the photosensitive material with the beam in a main scanning direction. A more detailed structure of such an apparatus is disclosed in Japanese Patent Laying-Open Gazette No. 63-158580 (1988), Japanese Patent Publication No. 62-19724 (1987), or Japanese Patent Laying-Open Gazette No. 5-27190 (1993) or No. 5-5846 (1993), for example. Recently, such a cylindrical inner surface scanner is applied to a direct prepress system called CTP. The CTP is an apparatus for preparing a printing plate by directly forming halftone dot images on a plate material on the basis of digital image signals integrating characters, figures and images, without forming a halftone dot film.
In the CTP, a plate material is directly scanned with a light beam to be exposed, as hereinabove described. Such plate materials include some types, and thicknesses (hereinafter referred to as plate thicknesses) of actually employed plate materials are over a range of 150 .mu.m to 300 .mu.m. Even if an imaging position by an imaging lens is set on a certain type of plate material, therefore, this imaging position is not located on a newly employed plate material if this plate material is of a different type, due to difference between thicknesses of the plate materials.
FIG. 20 typically shows this state. Referring to FIG. 20, an imaging position by an imaging lens is set on a plate material 61 having a plate thickness T. When the imaging lens set in this state is employed as such to scan a new plate material 62 of a plate thickness T' (T'&gt;T) with a light beam IL, an image of an out-of-focus state is inconveniently formed on the new plate material 62, and prescribed resolution cannot be obtained. Referring to FIG. 20, numeral 32 denotes an inner surface of a cylindrical inner surface drum, on which the plate materials 61 and 62 are mounted by vacuum mounting or the like.
Thus, it is necessary to control the position of an imaging lens so that its imaging position is regularly located on a plate material regardless of the type of the plate material. However, there has been implemented no CTP having means which can solve such a problem, since this problem has not been generally recognized in the CTP.
On the other hand, it may be necessary to set resolution of the same type of plate material at various values in response to halftone dot images. Thus, also strongly awaited is implementation of a cylindrical inner surface scanner for CTP having a function which can implement resolution values required under such circumstances one by one. In practice, however, there has been developed/implemented no cylindrical inner surface scanner also having such a function.